1. Field of the Invention
The present invention relates to a belt driving device and an image forming apparatus.
2. Description of the Related Art
A typical image forming apparatus, such as a multifunction product (MFP), a copier, a facsimile machine, and a printer, transfers an image formed on a latent-image carrier such as a photosensitive element to an intermediate transfer belt (primary transfer), and then transfers the image on the intermediate transfer belt to a recording medium (secondary transfer). In an image forming apparatus of a different type, an image formed on a latent-image carrier such as a photosensitive element is directly transferred to a recording medium held on a surface of a sheet-transfer belt. In the above image forming apparatuses, the image may be distorted if rotation speeds of the belts vary.
Japanese Patent Application Laid-open No. 2005-37620 discloses an image forming apparatus to prevent the image distortion due to the variation in the speed of the intermediate transfer belt. The image forming apparatus includes a plurality of rollers over which an intermediate transfer belt is supported (hereinafter, “supporting roller”). One of the supporting rollers serves as a driven roller that is driven by the rotation of the intermediate transfer belt, and the driven roller includes a circular detecting disk that is fixed to a rotation shaft of the driven roller. The detecting disk has a plurality of slits as marks and the slits revolve around the rotation shaft along with the rotation of the driven roller. A photosensor is provided near the driven roller, which serves as a mark detecting unit and detects each slot at a predetermined position. Time intervals between detections of the slits by the photosensor indicate the rotation speed of the intermediate transfer belt. By adjusting a driving speed of a driving motor serving as a driving source that drives the intermediate transfer belt based on a result of the detection by the photosensor, the rotation speed of the intermediate transfer belt can be kept stable.
One of the image forming apparatuses currently in use is configured to recognize a serial number, a property, and a size of each member for recognizing the end of a life of each member of the image forming apparatus and obtaining an excellent result by changing operation conditions based on variations in the property and the size per lot. For example, Japanese Patent Application Laid-open No. 2005-189599 discloses an image forming apparatus configured to read a barcode that is provided to a fixing belt of a fixing unit and that represents a serial number of the fixing belt, using a dedicated sensor for reading the barcode along with the rotation of the fixing belt. The number of rotations of the fixing belt is counted up until the result of reading the serial number changes, i.e., until the fixing belt is replaced, and when the result of the counting reaches the number representing the lifecycle of the fixing belt, the image forming apparatus recognizes that the life of the fixing belt is over. With this configuration, the image forming apparatus can automatically determine whether the life of the fixing belt is over; and therefore, it is unnecessary for a user to do a time-consuming operation for inputting information on the replacement of the fixing belt. As described above, because the image forming apparatus reads a serial number, for example, in a barcode that is different on a lot basis of a member, the end of the life of the member can be automatically recognized by the image forming apparatus.
Instead of the serial number, information on the property and the size of the product, which varies on a lot basis can be provided in the barcode. By changing operation conditions based on a result of reading the barcode, a better operation result can be obtained. A diameter of the driving roller that drives a belt member, which is an endless belt, can be slightly different on a product basis, which leads to a difference in the rotation speed on a product basis. By adjusting the rotation speed of the driving roller based on a result of reading a barcode representing the rotation speed of the belt member, it is possible to suppress variations in the rotation speed of the belt caused from the variation in the diameter of the driving roller. Moreover, although an electric resistance of each intermediate transfer belt of each image forming apparatus can be slightly different on a product basis, it is possible to suppress a variation in the electric resistance by adjusting a transfer bias based on the result of reading the electric resistance, thereby preventing an erroneous image transfer.
Regarding the image forming apparatus disclosed in Japanese Patent Laid-open No. 2005-37620, the supporting roller to which the marks (detecting disk) can be provided is limited to a specific type, which decreases a degree of freedom in layout. Specifically, at least one of the supporting rollers needs to be the driving roller that drives the belt member. The rotation speed of the driving roller does not accurately reflect the rotation speed of the belt member because a load is sometimes applied to the belt member and the belt member may slip on a surface of the driving roller. Therefore, it is desirable that the marks for detecting the speed be provided to the driven roller instead of the driving roller. However, some types of driven rollers, such as a tension roller that applies a tension to a belt member by a biasing member such as a spring, are not suitable for providing the marks. This is because the tension roller slightly moves when a force from the belt member rotating is applied to the tension roller, which leads to a variation in relative positions of the photosensor and the marks, and accordingly, it becomes difficult to accurately detecting the marks. As described above, because supporting rollers to which the marks can be provided for the speed detection are limited to the driven rollers other than the tension rollers, the degree of freedom in layout decreases.
Meanwhile, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2005-189599 includes a dedicated sensor for reading a barcode, which increases the cost.